Most rubber items contain textile reinforcing materials as an integral constituent to provide for dimensional stability and to reduce the high elongation of the rubber. Good adhesion of the rubber to the textile material is an indispensable prerequisite for satisfactory function and lengthy life of rubber items that contain textile reinforcing materials, for example motor vehicle tires, V-belts, and conveyor belts. If the adhesion is inadequate, the bond between the elastomer and the fiber material is broken with time, which results in destruction of the textile reinforcement from chafing, or by melting in case of local overheating. Rubber adhesion presents difficulties, particularly with the polyester fibers present as yarn filament, since there are hardly any mechanical anchoring possibilities for the rubber because of their molecular structure, as is the case with cotton fiber yarns, so that special binders are necessary.
While impregnation with resorcinol-formaldehyde resins combined with latices (RFL dip), especially vinyl-pyridine latex, is already sufficient to improve the adhesion of nylon yarns to rubber, special additional measures are necessary for polyester yarns. For them to provide adequate rubber adhesion with a conventional nylon dip (or with adhesive mixtures), the so-called spin-finish types of polyester were developed. To make them, specific adhesion promoters are applied to the polyester fibers immediately after they are spun, simultaneously with the spinning preparation, to improve rubber adhesion; they consist of definite epoxy compounds and amine hardeners, and impregnation is carried out on the cord yarn with an aqueous dispersion of resorcinol-formaldehyde resins and vinylpyridine latex. The drawbacks to applying epoxy compounds and amines consist on the one hand of the contamination of machine parts, and also of the fact that the production rates of polyester yarns are impaired, and furthermore, substantial environmental problems occur.
To avoid the application of adhesion promoters, it is known how to produce two-component yarns whose core consists of polyethylene glycol terephthalate and whose sheath consists of a polyamide (cf., for example, EP 0 398 221 A1), since polyamides by nature show better rubber adhesion than polyesters. However, this presents the problem that the adhesion of the polyester core to the polyamide sheath is inadequate. For this reason, it is necessary to reduce the core/sheath ratio of the fibers for practical application as tire cords in a way that results in insufficient utilization of the good and desirable polyester properties.
A problem underlying this invention was to avoid the procedural step of applying the aforementioned adhesion promoters, and to make available new polyester yarns made of core-sheath fibers with good rubber adhesion, for which there is no longer inadequate adhesion between core and sheath even with very large core/sheath ratios of the fibers.
This problem is solved by the features of the invention.